The extreme toxicity of radionuclides which decay by electron capture or internal conversion has been a subject of much interest for the past several years. These radionuclides, commonly referred to as Auger- emitters, are widely used in medicine and are found in the environment as well. The mechanism of radiation action of these radionuclides, when incorporated into tissue, is of great significance to basic science as well as diagnostic nuclear medicine, radiation oncology, and radiation protection in general. The advances made during the past few years are sufficiently important to warrant holding an international symposium to present and critically evaluate the recent experimental and theoretical data and set a course for future work in this field. This symposium will also provide a forum for the exchanging of techniques and ideas that are necessary for future development. It will also introduce and encourage young scientists to pursue careers in this field, the importance of which is being increasingly recognized. The publication of the proceedings by the American Association of Physicists in Medicine as a monograph will facilitate dissemination of the scientific information and will serve as an invaluable reference source.